In the production of certain workpieces, for example a hip-socket prosthesis, it is necessary to machine a part-spherical recess to a high degree of accuracy. The standard machine for such machining comprises a workpiece holder adapted to hold the workpiece, means for rotating the workpiece holder about a symmetry axis of the recess, a honing tool, and means for rotating the tool about a tool axis inclined to the workpiece axis and for simultaneously urging the tool along the tool axis against a surface of the recess to hone the surface.
To monitor the size of the recess as it is enlarged and trued, the apparatus further has a pair of feelers that symmetrically diametrally flank the workpiece axis and that are engageable with the surface of the recess. In many applications such a system is adequate, but in the machining of a hip-socket prosthesis, where the recess may have a diameter of 20 mm to 40 mm, very little can be tolerated by way of offsize. In fact such a hip-socket prosthesis must be machined to within at most 10 .mu.m of the desired diameter.
The known diameter gauges are only accurate when the two feelers can be exactly positioned on a diameter of the recess. It is not possible to do this with the known gauges since the tool is occupying or passing through the center of the recess. Thus accurate measurements can only be made when the tool is withdrawn, a procedure that interrupts the machining process and that makes it impossible to continuously monitor recess diameter.